The Exposure Core will be responsible for the assessment of all air pollutant exposures for all Projects of the Program Project. Air pollutant exposures will be assessed for each project following a three-tiered approach to characterize regional, community, and residential ambient particulate pollution. In the first tier, regional ambient concentrations will be characterized continuously for PM2.5, its major components, and other important particulate measures at a central stationary ambient monitoring (SAM) site located on the roof of Countway Library at 10 Shattuck Street, Boston, MA. At this site, ambient concentrations of PM2.5, PM10, black carbon (BC), sulfate (SO4[2-]), nitrate (NO3-), elemental (EC) and organic (OC) carbon, and particle counts (PC) will be measured continuously. Daily 24-h (9-9am) integrated PM2.5 samples will also be collected, with collected samples analyzed for PM2.5, BC (by reflectance), and elements (by XRF). Spatial variation in locally generated particle concentrations within metropolitan Boston will be assessed in Tier 2 using continuous measurements of ambient PM2.5 and BC concentrations at ten community monitoring stations. Finer spatial scale variation in locally generated particles will be captured in Tier 3 using residence-specific outdoor concentrations of traffic-related particles, which will be estimated using central and community site and State monitoring network pollution measurements and GIS-based spatial models. In our panel studies, exposures will also be measured directly for each study participant. Personal PM2.5 exposures will be measured continuously for each study participant for 24-h prior to each health measurement. Corresponding 24-h integrated personal PM2.5, SO4[2-] and BC exposures will also be measured for each participant. Personal measurements will include personal PM2.5, BC and particle count exposures, which will be measured continuously for participants as they travel on field trips aboard a diesel-powered bus. The Exposure Core is central to the Program Project. Not only will it provide all ambient and personal exposure measurements, but it will also provide expertise in atmospheric sciences, environmental and analytical chemistry, aerosol engineering and exposure assessment. This expertise will be used to characterize the sources and types of particle exposures and to provide an exposure assessment framework that will help to interpret and relate exposure and health effects data collected as part of this PPG.